Yes, you are right. However, it is generally used only in a positive light. Head over heels in love is probably the most common example. I can't think of any others really.

Yes, head over heels is slang to say you are overjoyed. This is a really interesting discussion on the origin of the word: https://www.quora.com/What-is-the-origin-of-the-saying-head-over-heels Head over heels is mostly used in talking about falling in love as an adjective - "She is head over heels for him."